I found beautiful ones at Bed Bath and Beyond...if you have that store in your area.
I bought some once with zips on from Germany, they soon ended up in the bin as zips on duvet covers tend to break really easily.
In europe, the duvet is the down comforter itself. Usually the duvet doesn't have quilting -- the lines of stitching that make separate compartments for the down. It's more like a bed-sized cotton bag filled with down.